39.6.2.7 Disabled Automatic Retransmission

According to the CAN Specification (see ISO 11898-1, 6.3.3 Recovery Management), the CAN provides means for automatic retransmission of frames that have lost arbitration or that have been disturbed by errors during transmission. By default automatic retransmission is enabled. To support time-triggered communication as described in ISO 11898-1, chapter 9.2, the automatic retransmission may be disabled via CCCR.DAR.

Frame Transmission in DAR Mode

In DAR mode all transmissions are automatically cancelled after they started on the CAN bus. A Tx Buffer’s Tx Request Pending bit TXBRP.TRPx is reset after successful transmission, when a transmission has not yet been started at the point of cancellation, has been aborted due to lost arbitration, or when an error occurred during frame transmission.

  • Successful transmission:
    • Corresponding Tx Buffer Transmission Occurred bit TXBTO.TOx set
    • Corresponding Tx Buffer Cancellation Finished bit TXBCF.CFx not set
  • Successful transmission in spite of cancellation:
    • Corresponding Tx Buffer Transmission Occurred bit TXBTO.TOx set
    • Corresponding Tx Buffer Cancellation Finished bit TXBCF.CFx set
  • Arbitration lost or frame transmission disturbed:
    • Corresponding Tx Buffer Transmission Occurred bit TXBTO.TOx not set
    • Corresponding Tx Buffer Cancellation Finished bit TXBCF.CFx set

In case of a successful frame transmission, and if storage of Tx events is enabled, a Tx Event FIFO element is written with Event Type ET = “10” (transmission in spite of cancellation).